The present invention relates generally to an exposure method for exposing an object in the photolithography, such as a single crystal substrate and a spherical semiconductor for a semiconductor wafer, and a glass substrate for a liquid crystal display (“LCD”), and more particularly to a method for optimizing an exposure dose and focus in an exposure apparatus.
Recent demands on smaller and higher-performance electronic apparatuses have increasingly required finer processing and more precise (sectional) shaping of a circuit pattern for a semiconductor to be installed in the electronic apparatus. The lithography technology for manufacturing the semiconductor device has conventionally utilized a projection exposure apparatus that projects and transfers a circuit pattern, which is formed on a reticle (or a mask), onto a wafer, etc. through a projection optical system.
The minimum size or resolution to be transferred by the projection exposure apparatus is proportionate to the wavelength of light used for exposure, and inversely proportionate to the numerical aperture (“NA”) of a projection optical system. Therefore, a method that attempts a high resolution uses a shorter wavelength of exposure light or increases the NA in the projection exposure apparatus.
The high resolution needs a proper exposure condition, when exposing a pattern on a reticle onto a substrate, such as a wafer onto which a photoresist (or a photosensitive agent, called “resist” hereinafter) is applied. The exposure condition includes various parameters, and particularly requires the exposure dose and focus to be appropriately set in order to transfer a fine pattern with a high resolution. The exposure dose in this specification is one parameter relating to the integral quantity of light irradiated upon a resist-applied wafer to be exposed. The focus is another parameter relating to whether or not a resist-applied wafer is placed on the best focus position at which a reticle pattern is imaged through a projection optical system in an exposure apparatus and, if not, how far the wafer is offset in the optical-axis direction in the projection optical system.
Again, the appropriately set exposure dose and focus are vital for the high resolution in the exposure apparatus. Accordingly, the exposure step sets the optimal exposure dose and the best focus for each processing or for each layer, and uses them for exposure. In addition, it is inspected whether a wafer is properly exposed with thus set exposure dose and focus, and the exposure dose and focus are corrected if necessary. See, for example, U.S. patent applications, Publication Nos. 2003/038250, 2003/106999, and 2003/121022.
A description will now be given of a method for setting the best exposure dose and the best focus, as disclosed in U.S. patent applications, Publication Nos. 2003/038250, 2003/106999, and 2003/121022, with reference to FIGS. 20 and 21. FIG. 20 is a flowchart for explaining a conventional method for setting the best exposure dose and focus. Referring to FIG. 20, a focus exposure matrix (“FEM”) wafer is formed so as to set the best exposure dose and focus for a test (or condition determining) wafer (step 1002). Here, the FEM wafer is one that has a FEM pattern as an aggregate of plural shots onto which patterns have been exposed with different exposure doses and/or focuses, as shown in FIG. 21. The shot is one exposure unit. FIG. 21 is an enlarged sectional view that illustrates sectional shapes in the shots in the FEM pattern formed on the FEM wafer. The abscissa axis denotes the exposure doses, and the ordinate axis denotes the focuses. FIG. 21 graphically shows pattern's sectional shapes as a result of the development of the pattern that has been exposed with each exposure dose and focus.
Next, a shape measuring apparatus (not shown), such as an optical CD measurer or a SEM, measures the FEM pattern's sectional shape formed in each shot on the FEM wafer (step 1004), so as to determine the best exposure dose and focus that can provide an intended resist pattern shape (step 1006). For example, in FIG. 21, an exposure dose E0 and a focus F0 used to expose a thick-framed pattern are determined as the optimal exposure dose and the best focus because they can maintain broad margins.
Turning back to FIG. 20, acquired after the optimal exposure dose and the best focus are determined is a correlation among each FEM pattern's sectional shape and exposure dose and focus for each exposed FEM pattern (step 1008). The memory or the like stores the correlation data for use with step 1016, which will be described later, so as to inspect the optimal exposure dose and the best focus for the mass-produced wafer.
A circuit pattern or the like is exposed onto the mass-produced wafer with the optimal exposure dose and focus calculated by the step 1006 (step 1012). Next, the sectional shape of the pattern exposed on the mass-produced wafer in step 1012 is measured (step 1014), and compared with that corresponding to the optimal exposure dose and focus stored in step 1008. Next follows calculations of offset amounts (and directions) between the actual exposure dose and focus and the optimal exposure dose and focus calculated in step 1006 (step 1016). Then, it is determined whether the offset amounts from the optimal exposure dose and focus fall within predefined permissible ranges (step 1018). When the offset amounts from the optimal exposure dose and focus fall within predefined permissible ranges, the step 1012 and subsequent steps are repeated to expose the mass-produced wafer, whereas when the offset amounts from the optimal exposure dose and focus do not fall within predefined permissible ranges, the offset amounts are fed back to the exposure apparatus to reset the exposure dose and focus (step 1012) to expose the mass-produced wafer.
While the conventional method sets and corrects the optimal exposure dose and focus based on the correlation among the pattern's shape, the exposure dose and focus for the mass-produced wafers, even setting and correcting to the optimal exposure dose and focus can no longer provide intended pattern's shape as a finer processing is required.
One cause rests in a difference between a FEM pattern forming process for a test wafer and an actual pattern forming process for a mass-produced wafer. The difference between these two pattern forming processes can cause errors in calculating the offset amounts from the best focus and exposure dose in the exposure apparatus at the time of the mass production. The calculation is based on the deforming shape measuring marks on the mass-produced wafer, and shape information of the FEM pattern on the test wafer.
The conventional method determines conditions on the rough premise that when the same type of resist is used for both the test wafer and the mass-produced wafers, they have the common optimal exposure dose and focus conditions. No careful attentions have not been paid as to how the different forming processes affect the best exposure dose and focus conditions.
This is required from apparatus operators' demands on a longer operating period of time of an expensive semiconductor exposure apparatus. The longer operating time period inevitably limits the test time period. Thus, this demands have introduced an assumption that the test wafer and the mass-produced wafer have the common best exposure dose and focus when some common major parameters are used.
However, the processing of the exposed circuit pattern, which is required to be finer and finer, can no longer allow ignorance of a conventionally ignorable, slight difference in forming process between the test wafer and the mass-produced wafer.
Another cause of a deformed pattern irrespective of the best exposure dose and the best focus is an immature optimization in an approach for acquiring the relational equation that represents a relationship between the shape measuring pattern's shape arranged on each shot that forms the FEM pattern and the exposure conditions under which each shot is exposed.